Why wont my FaceTime work?

Me and my friend both have iphone 4's. We both have verizon, we both have wi-fi, and we both have face time. When I try to Face Time her, it says connecting, but she never receives it. When she Face Times me, I never receive it. We have Face Timed once before. I checked my settings, everything is on, she checked her settings, everything is on. I restored my iphone, I tried everything. I tried to Face Time other friends, it works, but when I face time her, it doesn't work, why is this?